1. Field of the Invention
The present invention relates to a method and apparatus for handling reset in a wireless communications system, and more particularly, to a method and apparatus for accurately handling protocol errors, enhancing transmission efficiency, and preventing unnecessary PDU discard.
2. Description of the Prior Art
The third generation (3G) mobile communications system has adopted a Wideband Code Division Multiple Access (WCDMA) wireless air interface access method for a cellular network. WCDMA can provide high frequency spectrum utilization, universal coverage, and high quality, high speed multimedia data transmission. The WCDMA method also meets all kinds of QoS requirements simultaneously, providing diverse flexible two-way transmission services and better communication quality to reduce transmission interruption rates.
The Access Stratum (AS) of the 3G mobile communications system comprises various protocol stacks, such as Radio Resource Control (RRC), Radio Link Control (RLC), Media Access Control (MAC), Packet Data Convergence Protocol (PDCP), and Broadcast/Multicast Control (BMC). In addition, the 3G mobile communications system also provides different levels of transmission quality, and can operate in different corresponding modes according to different transmission quality requirements, such as: Transparent Mode (TM), Unacknowledged Mode (UM), and Acknowledged Mode (AM).
A protocol layer, such as RLC or PDCP, consists of a transmitting side and a receiving side. The transmitting side and the receiving side each process transmission and reception through the RLC layer, and both sides can share system resources. For example, uplink (UL) transmission is performed by the transmitting side of a UE (user equipment) and the receiving side of the network, while downlink (DL) transmission is performed by the receiving side of the UE and the transmitting side of the network.
In order to maintain data accuracy in AM, when a protocol error is detected, a reset procedure is executed, and a principle for determining the protocol error is according to whether the RLC entity of the transmitter detects one of the three following conditions:
Condition 1: “No discard after MaxDAT number of transmissions” mode is configured, and a variable VT(DAT) is equal to a parameter MaxDAT, and then the RLC reset procedure will be executed. The variable VT(DAT) indicates a number of times an AM PDU (Acknowledgement Mode Protocol Data Unit) has been scheduled for transmission, while the parameter MaxDAT indicates an upper limit of VT(DAT).
Condition 2: A variable VT(MRW) is equal to a parameter MaxMRW. The variable VT(MRW) indicates a number of times a Move Receiving Window (MRW) command has been transmitted, while the parameter MaxMRW indicates an upper limit of VT(MRW).
Condition 3: A STATUS PDU reported by the receiver to the transmitter or a piggybacked STATUS PDU contains an erroneous sequence number, such as a sequence number reported as missing that has already been acknowledged as received, or a sequence number reported as received that has not yet been transmitted by the transmitter.
Note that, all the three conditions are events on the transmitting side of an RLC entity. For UE, the events are on the uplink, and for the network, the events are on the downlink. After the reset procedure is triggered, the transmitting side and the receiving side are reset, data transmission are stopped, DL HFN (Hyper Frame Number) and UL HFN values are exchanged between the peer entities, PDUs in the receiving side are discarded, SDUs that were transmitted in the transmitting side are discarded, state variables are reset, and then the data transmission restarts. Therefore, the RLC reset procedure is time consuming and it causes data loss.